1. Field of the Invention
The present invention relates to a bandpass filter using dielectric resonator which is used to a mobile radio communication base station such as a cellular mobile telephone, a personal communications service (PCS) and a wireless local loop (WLL), more particularly to a bandpass filter which is transmitting to a few loss signals which lie in a desired frequency band while intercepting all the frequencies outside the desired frequency band by forming the stepped dielectric resonators, and a bandpass filter having a variable notch cable outside the filter to show a desirable attenuation characteristic.
2. Description of the Related Art
Generally, a bandpass filter is the parts used at the mobile radio communication base station such as a cellular mobile telephone, a personal communications service (PCS) and a wireless local loop (WLL), and a radio frequency (RF) band. The role which a bandpass filter is to fulfill is transmitting to a few loss signals which lie in a desired frequency band while intercepting all the frequencies outside the desired band.
A conventional bandpass filter described above has been used to radio-based communications systems operating in the microwave range. FIG. 1B is a perspective view showing a conventional bandpass filter, FIG. 1C is a top view of FIG. 1B.
As shown in FIG. 1B and FIG. 1D, a bandpass filter comprises a metallic housing 12 formed by a plurality of cavities, a dielectric resonator 11 installed in the cavities each of the housing 12, an input/output connector 13 installed on the both side end of the housing 12, a coupling loop 15 combined with the input/output connector 13, a partition 14, which has windows 14a for combining resonance mode forms a boundary among cavities, frequency control plate 16, and tuning bar 17.
FIG. 1A is a perspective view showing a dielectric resonator using a bandpass filter.
As shown in FIG. 1A, a uniform dielectric resonator 11 is formed to a cylinder shape. The filter using uniform dielectric resonators involves the needless signals by resonating not only the fundamental mode (TE.sub.01.delta.) but also the higher-order mode. Accordingly, the filter having uniform dielectric resonators has a bad effect on a communications system by needless signals, which is resulted from the higher-order mode, in the neighborhood of the fundamental mode by the higher-order mode.
Also, it is extremely necessary to have a bandpass filter showing high quality coefficient (Q) in the low band region and low insertion loss in the pass band region. In most of the cases, the attenuation characteristic of the specified region to decrease interference between the neighboring channels and the transmitter/receiver bands and must be excellent.
In this case, a conventional method is to use the dielectric resonator having the high quality coefficient. However, this method is not only difficult to accomplish, but also involves a high manufacturing cost. To improve the attenuation characteristic, a conventional bandpass filter has been proposed to install a notch cable in the housing.
FIG. 2A is a perspective view showing a bandpass filter using conventional dielectric resonators. FIG. 2B is a top view of FIG. 2A.
As shown in FIGS. 2A and 2B, when RF signal is applied, the propagation is induced by the first dielectric resonator 21' through a coupling loop 25. The signal power through the window 24a of a partition which is controlling a coupling capacity of the signal power and a band width is transmitted to the second dielectric resonator 21". By the same method, Signals of the desired frequency band are transmitted to the output connector 23'. At this time, the higher attenuation is generated in the specified band region by a notch cable 26 inserted into the housing 22. Symbol 27 is a housing cover.
However, above described method decreases a quality coefficient (Q) and increases a loss, because the notch cable changes the inside structure of the filter. Also, transformation and reestablishment after manufacturing of the filter is impossible. The needless wave may arise at certain frequency because of generating another resonance mode by the inserted notch cable 26, also the wave may be distorted by changing the electromagnetic shape in course of resonance.